Oroville Spillways Recovery

The Oroville main spillway and emergency spillway.

The Oroville main spillway and emergency spillway site. October, 2018 (DWR)

Starting in May 2017, DWR and its construction contractors began repairing and rebuilding Oroville’s main and emergency spillways. As of November 1, 2018, the main spillway has been successfully reconstructed, meeting DWR’s public safety construction milestone. 

Work on the emergency spillway will continue into 2019 to complete a concrete buttress to further bolster the emergency spillway weir. This work, in addition to an underground secant pile wall and splashpad on the hillside below will prevent uphill erosion if the emergency spillway is ever used again.

More than 1,000 people have worked more than 2 million hours to rebuild the Oroville spillways to ensure the safety of downstream communities.

Butte County Sheriff
The Butte County Sheriff’s Office played an important role in the emergency response during the Lake Oroville spillways incident. Along with 24-hour policing, the Butte County Sheriffs also operates civil services, corrections, and court security in Butte County.

California Department of Parks and Recreation
The California Department of Parks and Recreation, also known as the California State Parks, operates the Oroville State Recreation Area around Oroville Dam and the Lake Oroville spillways. The California State Parks maintains the “dam cams,” airing a live feed of the construction at the Lake Oroville spillways.

California Department of Fish and Wildlife
California Fish and Wildlife officials manage the Feather River Fish Hatchery, located just downstream from Oroville Dam. Fish and Wildlife staff at the hatchery help monitor migratory spawning for salmon, and assists DWR with maintaining the Feather River.

Butte County Fire Department
The Butte County Fire Department is one of the largest fire departments in Northern California. CAL FIRE is a major component of the Department. Butte County Fire Departments serves the large unincorporated areas of Butte County with 23 staffed fire stations and 14 volunteer fire stations.

Oroville Police Department
The Oroville Police Department is the local law enforcement agency in Oroville. Oroville PD consists of patrol and K9 units, school resource officers, and municipal law enforcement officers.

California Highway Patrol
The California Highway Patrol is one of the law enforcement organizations tasked with maintaining public safety at the Lake Oroville spillways. CHP assisted state and local agencies during the Lake Oroville spillways incident and works closely with partner agencies to secure the construction site.

Butte County Office of Emergency Management (OEM)
During disaster incidents, the Butte County OEM coordinates the county response through the Emergency Operations Center (EOC). When activated, the EOC provides a central location for responding and supporting agencies to collaborate response and recovery efforts. In non-disaster times, the Butte County OEM supports and coordinates disaster planning, community preparedness, mitigation, and training.

The California Department of Transportation (Cal Trans)
The California Department of Transportation, Cal Trans, is the primary agency responsible for the state’s highways, roads, and public transportation.

Oroville Fire Department
The Oroville Fire Department is one of the oldest fire departments in California and serves the city of Oroville.

Butte County Public Works
Butte County Public Works maintains country roads, bridges, and land management operations.

Public Gas & Electric (PG&E)
PG&E provides gas and electricity for much of California and operate the powerlines and electrical equipment at the Oroville Dam complex.

California Department of Forestry and Fire Protection (CAL FIRE)
CAL FIRE is the state agency responsible for fire protection in State Responsibility Areas of California, as well as the administration of the state's private and public forests. CAL FIRE also provides varied emergency services in 36 of the state's 58 counties via contracts with local governments.

United States Army Corps of Engineers (USACE)
The USACE consults DWR on flood storage capacity at Lake Oroville. USACE owns and operates dams and canals throughout the United States, and is one of the largest public engineering, design, and construction management agencies.

Federal Energy Regulatory Commission (FERC)
FERC partners with DWR on the management of the Oroville Dam complex. FERC is a federal agency that regulates the transmission and sale of electricity and natural gas, and regulates the transportation of oil by pipeline. FERC also reviews proposals to build interstate natural gas pipelines, natural gas storage projects, and liquefied natural gas (LNG) terminals, and licensing for non-federal hydropower projects.

Oroville Hospital
Oroville Hospital is a Level III trauma care center and hospital in Oroville, California, founded in 1962.

American Red Cross
The American Red Cross is an international aid organization that assists with disaster relief, blood donation, training and certification for health and safety. The Sierra-Delta Chapter and the Northeastern California Chapter serve northern California.

California Conservation Corps
The California Conservation Corps (CCC) is the oldest and largest conservation corps in the country and develops thousands of young men and women into citizens with character, credentials and commitment.

California National Guard
The California Army National Guard organizes, trains, equips, and resources community-based land forces. On order, they mobilize to support state and/or federal authority. The California Army National Guard mobilized 500 troops to help with emergency services during the Lake Oroville spillways incident in February.

Bureau of Indian Affairs (BIA)
The Bureau of Indian Affairs (BIA) is the oldest bureau in the United States Department of the Interior and is responsible for the administration and management for American Indians, Indian tribes, and Alaska Natives. The state of California has four BIA offices.

Governor's Office of Emergency Services (CAL OES)
The Governor’s Office of Emergency Services, or CAL OES, oversees and coordinates emergency preparedness, response, recovery and homeland security activities within the state.

Gridley-Biggs Police
The Gridley-Biggs Police Department is a joint law enforcement department shared between the cities of Gridley and Biggs.

The terminology and organizations listed are meant to educate readers and share commonly used terms associated with the Lake Oroville Spillways Emergency Recovery Project.


Abutment: The substructure at the ends of a dam on which the superstructure contacts or rests.

Afterbay: A reservoir, stream or pond that is directly released from the turbines of a hydroelectric powerplant. The Thermalito Afterbay serves as an off-stream reservoir for Lake Oroville, the Feather River and agricultural irrigation.

Asbestos: A naturally occurring mineral in rock and soil.


Backfill: The excavated material that is used to refill a hole.

Bathymetry: A measurement of the depth of water similar to topography.

Bedrock: A solid rock underlying loose deposits such as soil.

Bore: A term for drilling into rock, concrete, or soil.


California Environmental Quality Act (CEQA): A statute passed in 1970 that requires a statewide environmental protection policy. Under CEQA, state and local agencies must follow a protocol of analysis and public disclosure of environmental impacts of proposed projects and adopt all feasible measures to mitigate those impacts.

Canal: An artificial waterway constructed to transport water. The Thermalito Power Canal moves water from the Thermalito Diversion Dam to the Thermalito Forebay for power generation.

Cavitation: Cavitation is the formation of vapor cavities in a liquid, most observable as bubbles.

Central Valley Project (CVP): The federal water management project that is operated by the Bureau of Reclamation and covers the Cascade Mountains near Redding in the north to the Tehachapi Mountains near Bakersfield in the south. It consists of 20 dams and reservoirs, 11 powerplants, and 500 miles of major canals, conduits, tunnels, and related facilities.

CFS: Stands for cubic feet per second, a standard unit of measurement for flow rate of water. One cubic foot is about 7.5 gallons.

Controlled blasting: The controlled use of explosives or gas pressure blasting to break rock for excavation. Also used for slope set back. Controlled blasting is a common practice around the world to remove rock.

Contingency plan: The course of action designed to help an organization respond effectively to a possible event.

Crest: The highest point of a spillway or dam.

Critical Energy Infrastructure Information (CEII): Critical Energy Infrastructure Information means specific engineering, vulnerability, or detailed design information about proposed or existing critical infrastructure (physical or virtual) that:

- Relates details about the production, generation, transmission, or distribution of energy;
- Could be useful to a person planning an attack on critical infrastructure;
- Is exempt from mandatory disclosure under the Freedom of Information Act; and
- Gives strategic information beyond the location of the critical infrastructure.

Crusher plant: An installation used to crush rock.

Cut-off wall: A wall of impervious material like concrete or steel that is built to reduce seepage and prevent erosion. A cut-off wall will be constructed underground about 730 feet downstream of the emergency spillway to prevent uphill erosion. Also known as a “secant pile wall.”


Dam: According to California Code, Water Code - WAT § 6002, means any artificial barrier, together with appurtenant works, which does or may impound or divert water, and which either (a) is or will be 25 feet or more in height from the natural bed of the stream or watercourse at the downstream toe of the barrier, as determined by the department, or from the lowest elevation of the outside limit of the barrier, as determined by the department, if it is not across a stream channel or watercourse, to the maximum possible water storage elevation or (b) has or will have an impounding capacity of 50 acre-feet or more.

Delamination: The process of material separating from repeated stress. Stress over time can cause eventual failure of the material.

Dental concrete: A conventional concrete used to shape surfaces, fill irregularities and protect poor rock.

Dentate: See Energy Dissipater.

Discharge: Usually applied to measure the rate of flow of water traveling through a channel or pipe at a given instant.

Drawdown: The lowering of the water level in a reservoir or a downstream levee.


Earthfill or earthen dam: Classified when compacted earth layers comprise a majority of the total volume. Oroville Dam is the largest earthen dam in the United States.

Emergency Spillway: An appurtenant structure of a dam complex that is only used when reservoir inflows exceed the amount that other structures – flood control spillways, powerplant, river valves – can release water before the dam overtops.

Emergency Spillway at Lake Oroville: A concrete weir to the right of the main spillway that provides relief for the reservoir if water levels exceed 901 feet.

Energy dissipater: A structure that slows fast-moving water to prevent erosion. Energy dissipaters are also called dentates.

Energy dissipation: Energy dissipation is a physical process by which energy becomes unavailable. In the context of water and dams, there are various types of structures and methods to slow down or absorb the force of water as it is released from a dam.

Evacuation notice: An announcement to the public from state and local officials to leave an area that is in current or potential danger. Often time, these notices are decided by law enforcement in the interest of public safety.


Feather River: Principal tributary of the Sacramento River in the Sacramento Valley of Northern California. The main part of the Feather River is 71 miles long.

Foliation: Shearing and differential pressure that creates layered “bands” or designs in metamorphic rock.


Gated spillway: A controlled spillway with gates that regulate the flow of water. The gated spillway at Oroville Dam is also called the “main spillway.”


Head cut: An erosional feature where an abrupt vertical drop occurs at the leading edge of a gully. This can resemble a small tidal pool or a large waterfall and is also known as a “knickpoint.”

Herringbone drain: A pattern that has one or two vertical drains which have diagonal drains running into them.

Hydroelectric Powerplant: Located at the base of a dam or reservoir. Provides power by capturing the kinetic energy of fast-moving water through gravity or by pumping large amounts of water past turbines. The Edward Hyatt Powerplant is a hydroelectric powerplant at the base of Oroville Dam. The Thermalito Diversion Dam Powerplant is located at the base of the Thermalito Diversion Dam.


Inclinometer: An instrument that measures angles of slope.



Kinematic analysis: The study of motion based on various properties like distance and speed. Kinematics help engineers determine the stability or rocks, slopes and possible areas of failure.


Lake Oroville: One of California’s largest man-made lakes, located where the North, Middle, and South Forks and the West Branches of the Feather River join. The lake and the surrounding State Recreation Area provide access to numerous recreational opportunities including fishing, boating, camping, swimming, water skiing, hiking, and hunting.

Lake Oroville Spillways Emergency Recovery Project: Name given to the planning and construction of the spillways. The objective of the recovery project is to ensure that by November 1, the Lake Oroville Spillways can safely pass Feather River watershed flows. Construction on the main spillway and emergency spillway will continue into 2018.

Lake Oroville Spillways Emergency Response: Name for the operations and management during and directly after the spillways incident. The goal of the response was to ensure the safety of residents downstream and Oroville Dam facilities. This included reinforcing the damaged main spillway so it could functionally release water, reinforcing the emergency spillway, removing nearly two million cubic yards of debris from the Diversion Pool, stabilizing powerlines and maintaining the use of Hyatt Powerplant.

Layback: The amount of material that must be excavated for the slope of a pit wall to be at a safe angle for construction activities to occur.

Levee: An elongated ridge, natural or artificially constructed, which contains water flows. In California, levees are man-made to manage floodwaters.



Nappe: A large body or sheet of rock that has been moved by faulting or folding.


Oroville Dam: Oroville Dam is the tallest dam in the United States, measuring 770 feet high and 6,920 feet across. Construction is occurring on the main and emergency spillways, not on Oroville Dam itself.

Oroville Wildlife Area: The primarily riparian woodland habitat along the Feather River and grasslands around the Thermalito Afterbay.


Piezometer: An instrument used in dams for measuring the pore water pressure in a soil medium. Over time, some piezometers are expected to stop working.

Portland Cement Concrete (PCC): A construction material generally used as a binding agent. It is composed of cement and other materials, such as fly ash and slag cement, aggregate (gravels, crushed limestone or granite or fine sand) and water. Also known as “Plain Cement Concrete.”

Potential Failure Mode Analysis (PFMA): Determinations of all the different ways a dam can fail. PFMA’s are an important part of dam safety programs and encourage better monitoring and risk assessment.



Reinforcing bar (rebar): A steel bar or mesh of steel wires used as a tension device in reinforced concrete and reinforced masonry structures.

Riprap: Loose stone used to form a foundation for a breakwater.

River bank: A sloped side of a river acting as a barrier between the water and level ground.

River Valve Outlet System: Releases water from the bottom of the reservoir into the diversion pool and is a key component of maintaining cooler river temperatures for certain species of fish. The system is contained under the Oroville Dam inside the Edward Hyatt Powerplant.

Roller-compacted concrete (RCC): Ultra-tough, zero-slump concrete used in many dams and spillways. RCC often times makes up the entirety of a structure because it is more affordable, efficient, and dries quick.


Scour: Holes caused by swiftly moving water that can compromise the integrity of a structure.

Secant pile wall: Constructed underground about 730 feet downstream of the Lake Oroville Emergency Spillway to prevent uphill erosion. Also called a “cut-off wall.”

Seepage: The expected movement of water that may take place through a levee, dam, foundation or abutments. Lake Oroville Dam has one of the lowest acceptable amounts of seepage for an earthen dam.

Seismometer: Measures seismic waves and the motion of the ground. Most commonly used to measure earthquakes.

Shotcrete: A cement-sand mortar that is sprayed onto formwork, walls, or rock by a compressed air ejector giving a very dense, strong concrete layer.

Spalling: The result of water or moisture entering brick, concrete, or natural stone and forcing the surface to peel, pop out, or flake off.

Spillway: A structure used to provide the controlled release of flows from a dam or levee into a downstream area.

Spillway slab: A spillway slab is the flat concrete sections of the main spillway that water moves across as it is released from the radial gates.

Spillway walls: Vertical concrete walls along either side of the main spillway that keep water releases contained until they are deposited into a diversion pool or river.

State Water Project (SWP): The SWP is a water storage and delivery system consisting of 34 storage facilities, reservoirs, and lakes, 20 pumping plants, 4 pumping-generating plants, 5 hydroelectric powerplants, and about 701 miles of open canals and pipelines that provides supplemental water to approximately 25 million Californians and about 750,000 acres of irrigated farmland.


Tailrace: A channel into which water is discharged after passing through turbines.

Tailwater: Waters located directly downstream from a dam, hydro powerplant or culvert that fluctuate with water releases. The Thermalito Diversion Pool serves as the tailwater for the Oroville Dam.

Thermalito Diversion Dam and Pool: Diverts Feather River water into the Thermalito Power Canal for power generation at Thermalito Pumping Generating Plant about 4.5 miles downstream from Oroville Dam.

Turbidity: Turbidity is the cloudiness of a liquid caused by large numbers of individual particles that are generally invisible to the naked eye. Turbidity impacts water quality and ability of some fish species to survive.


Upstream slope: The part of the dam that is in contact with reservoir water.


Vegetation Area: The vegetation area on the face of the Oroville Dam has been monitored for nearly five decades on a regular basis and does not represent a threat to the integrity of the dam. It is a seasonal wet spot resulting from temporarily trapped rainwater, requiring several months to drain, which allows vegetation to grow during that time.


Waterstop: An element of a concrete structure, intended to prevent the passages of fluids (such as water) from running through concrete joints.

Weir: A horizontal barrier that water usually flows over. Weirs are not dams because water flows over or through them, but they do assist with altering and measuring waterflows.





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Email: Oroville@water.ca.gov
Call: (916) 653-6192

Oroville Spillways News

The Department of Water Resources (DWR) and California State Parks today announced that Lake Oroville State Recreation Area’s Spillway Boat Ramp area will remain open for the Labor Day holiday on September 2, 2019. The Spillway Boat Ramp opened to the public on August 9 after two years of closure for the Oroville Dam Spillways Reconstruction Projec ...


The Department of Water Resources (DWR), California Highway Patrol (CHP), California State Parks and the Butte County Sheriff’s Office today announced that Oroville Dam’s Spillway Boat Ramp will reopen on a partial schedule to the public on Friday, August 9.



Oroville construction YouTube video playlist

Click the image above to view all of DWR's Oroville spillways construction videos on our YouTube playlist.